1. Field of the Invention
The present invention relates to an electrophotographic color image forming apparatus that includes a plurality of photosensitive members, such as a laser printer, a copier, and a facsimile machine.
2. Description of the Related Art
There exists a color image forming apparatus that uses a system of sequentially forming toner images of four colors (yellow (Y), magenta (M), cyan (C), and black (K)) on a single photosensitive member and sequentially transferring and superimposing the toner images on a transfer member (hereinafter referred to as 4-pass system). Image forming using the 4-pass system is disadvantageous in that it takes a long time to acquire a final color image.
With the increase in image forming speed, a color image forming apparatus using an in-line system of emitting light beams from a plurality of optical devices to individually scan a plurality of photosensitive members is becoming known.
The color image forming apparatus using the in-line system forms toner images corresponding to four colors on a plurality of photosensitive members using a plurality of developing units, superimposes the four color toner images on an intermediate transfer belt, and finally transfers the combined toner image to a sheet. Because this in-line image forming apparatus forms four color toner images at a time, the time required for acquiring a final color image can be shorter than that required in the image forming apparatus using the 4-pass system.
However, unlike the 4-pass image forming apparatus, because using a plurality of photosensitive members and a plurality of optical devices, the in-line image forming apparatus tends to have color shift of a periodically varying AC component resulting from decentering of a gear for driving each photosensitive member or unevenness of rotation of a motor. To address this AC-component color shift, there is a known measure to suppress relative color shift by adjusting the relationship between rotational phases of photosensitive drums corresponding to colors to a desired state. One such example is described in Japanese Patent Laid-Open No. 2004-233952 (hereinafter referred to as Patent Document 1).
Various ideas to address the AC-component color shift have been proposed.
For example, Patent Document 1 describes a technique of, in full-color mode, stopping a photosensitive member for use in color printing (hereinafter referred to as the color photosensitive member) and a photosensitive member for use in black printing (hereinafter referred to as the black photosensitive member) at a position different from the position where each of the photosensitive members starts its rotation while maintaining the relationship between the color and black photosensitive members at a phase relationship at which color shift is small.
With this technique, the engagement relationship between a gear for use in color printing and a gear for use in black printing is made different from a preceding one, and local degradation of the gears is reduced. The position at which each of the photosensitive members stops is also changed successively, and local degradation therein is also reduced. Patent Document 1 also describes that an advantage of preventing or effectively reducing color shift in a color image is obtainable because the photosensitive members are activated while a predetermined rotational phase relationship between the color and black photosensitive members is maintained.
In the technique described in Patent Document 1, the rotational phase relationship between the black and color photosensitive members is maintained at a constant state by stopping the black photosensitive member at the same position as the position at which its rotation starts in monochrome mode even if the mode is switched between the monochrome and full-color modes. Patent document 1 describes that this leads to an advantage of, even if an image is formed in black mode, preventing or effectively reducing color shift in a color image formed in subsequent printing in full-color mode.
However, the above image forming apparatus has problems described below.
First, if monochrome printing is repeatedly performed, the black photosensitive member stops at the same position as the preceding stop position again and again. This leads to local abrasion in the gear for the black photosensitive member.
In addition to the gear, contacts on the surface of the photosensitive member, including a blade of the photosensitive member and a developing roller, are always the same, so the photosensitive layer may also suffer from local abrasion. If the abrasion in the photosensitive layer becomes worse, the sensitivity to exposure becomes lost or it becomes impossible to uniformly charge the surface of the photosensitive member at a desired potential. This leads to difficulty in forming a high-quality image.
Second, because, in color mode, the color and black photosensitive members are activated at the same time, as described above, a large starting current is necessary. More specifically, a starting current for a motor for driving the color photosensitive member and that for a motor for driving the black photosensitive member overlap each other, so a large current must flow. This results in necessity of using a large capacity part in a power supply and other electrical components, so the cost is increased.
Even in the technique disclosed in Patent Document 1, if each of the color and black photosensitive members stops at a position different from the preceding stop position each time and both of the photosensitive members are activated separately, the above problems will be solved.
However, this approach will be unable to maintain the phase relationship between the color and black photosensitive members, so it will be difficult to retain the phase difference at a desired state on starting, which is an original aim of Patent Document 1.
Under the circumstances, it is to be desired that even if black mono-color printing is continuously performed local abrasion in a rotary member or a gear for driving the rotary member be reduced, an overlap between peaks of the starting currents of motors for driving rotary members be reduced, and the phase relationship between the rotary members become a desired one.